1. Technical Field
The invention relates to methods for detecting joints between workpieces, and also to an apparatus for detecting a joint between workpieces. The invention further relates to apparatus and methods for seam tracking.
2. Background Information
When materials are joined together, particularly by welding though also by adhesive bonding, the problem of detecting the joint arises. This is true in the case of both spot joints (spot welds, spot bonds) and linear joints (weld seam, adhesive bead). Detection may take place both before and after the joining process, and is here understood as meaning on the one hand ascertainment of the position, and on the other hand ascertainment of the condition or quality, of the joint. Thus welding processes with a small heat-affected zone (beam welding, in particular laser beam welding) in which the parts to be joined are butted together require a seam tracking system to detect the position of the joint in order that the beam can be made to follow exactly the dividing line between the workpiece components during welding. Especially when welding butted together metal sheets to form “tailored blanks” (Platinen), it is necessary to have very precise beam tracking to detect the dividing line between the workpieces even when the components are cut exactly to size and the gap between the workpieces is very small or non-existent. This also needs to be assured when sheet-metal parts of equal thickness are welded together, so that no jump in thickness occurs at the dividing line as happens when sheets of different thickness are joined together. It is also necessary that edge damage to the parts should be detected, as it is not possible to obtain a high-quality weld seam between edges that are damaged.
An evaluation of the joint also needs to be carried out after the joining process. For example, a weld seam needs to be evaluated as to quality and/or for the presence of weld defects. Laser-welded tailored blanks which are formed into shaped parts, and also tubes, especially tubes made from sheet-metal components and intended for hydroforming-so-called tailored tubes should be subjected to a 100% inspection of the weld seam.
High rates of travel and field conditions have led to the use of contactless detection systems. These must reliably detect the edges of the tailored blanks for welding, even under the conditions which have been mentioned, and must monlor the misalignment and edge quality of the tailored blanks; and they must also measure geometrical data such as convexity, concavity and edge misalignment to ensure that ISO limits are not exceeded. And besides geometrical data, local defects like porosity, small holes and incomplete penetration should also be found in order that the quality of the seams can be assured.
To distinguish the edges during detection of position, it is customary to use the so-called light section method in which a light beam, e.g., a laser beam, is placed across the joint line and its offset or change of direction is detected. In VDI Berichte No. 1572, 2000, P. Dillinger, A. Horn, K.-H. Noffz, High-speed detection of geometry in laser welding by means of apparatus for detecting a joint between workpieces, field programmable gate array (FPGA) processors, a laser line is placed across the weld seam and recorded by means of a complimentary metal oxide semiconductor (CMOS) camera. To place and evaluate light sections at short intervals, a scanning frequency of 500 Hz is adopted. Visual inspection of the windows for the laser lines is performed with a grey-level image under incident illumination, but the image is produced only in strips and therefore has a scanning frequency of only 16 Hz, and, as stated, serves only as a check of the light section process.
There is a commercially available sensor system for detecting the dividing line between butted-together parts ahead of the welding point (TRUMPF TNS seam sensing system, TRUMPF Lasertechnik GmbH) that uses a light section projector and a charge coupled device (CCD) camera, a video image again being recorded with incident illumination. The video image is evaluated according to brightness signal and its derivative to determine the position of the butt joint, which can also be performed by the light section method. For matt workpiece surfaces, it is suggested that the incident-illumination image and the light section projection should be superimposed, so that the lateral position of the butt joint can be determined by grey-level image evaluation and the vertical misalignment can be detected by light-section evaluation. DE-A4312241 likewise describes a position detection by the light section method and grey-level evaluation of an image obtained by a two-dimensional CCD array, with light-section and grey-level evaluation conducted intermittently. It is also known to use the light section method for detecting the joint, such as the weld seam, after the joining process.